1. Field of the Invention
The inventors have studied a method for selecting a serving call session control function unit within an “Internet Protocol Multimedia Subsystem” based service communication system in order to register a user of a mobile communication terminal for at least one communication service within a mobile communication system, wherein the IMS-based service communication system has several serving call session control function units and at least one interrogation call session control function unit and the mobile communication system has at least one policy decision point unit.
2. Description of the Related Art
In existing second generation mobile communications systems, communication services such as, for example, GPRS (“General Packet Radio Services”) are already available where data is transmitted as data packets. This enables better utilization of the transmission resources available in the associated mobile communications systems or mobile communication systems.
In order to extend the existing GPRS architecture and to provide additional multimedia communication services, in particular for third generation mobile communication systems, for example, communication systems designed according to UMTS (“Universal Mobile Telecommunication System”) transmission technology, there are optimized special communication systems or service communication systems, such as, for example, the “Internet Protocol Multimedia Subsystem” (IMS) communication system.
The service users are connected to the IMS-based service communication system via an access communication system, for example a GPRS access communication system and then the IMS-based service communication system provides data or multimedia services via the existing connection. Here one could mention voice services, data services, audio services, video services, information services and program communication services as examples of communication or multimedia services.
Within such an IMS-based service communication system there is a “Session Initiation Protocol” (SIP) signaling protocol for setting up communication links. The communication subscribers are identified within the IMS-based service communication system using long-term stable identifiers, so-called “SIP Uniform Resource Identifier” (SIP URI), the construction of which is similar to that of a standard e-mail address.
In order to implement the SIP signaling protocol, there are special service control units that provide the link and service control functions, the so-called “Call State Control Function” (CSCF). An IMS-based service communication system of this kind comprises, for example, a “Serving Call Session Control Function” (S-CSCF) unit, an “Interrogation Call Session Control Function” (I-CSCF) unit and a “Proxy Call Session Control Function” (P-CSCF) unit, which take on different signaling and control tasks.
The selection of the S-CSCF unit to be provided to create a communication service is currently made in accordance with the selection procedure described in the Standard 3GPP TS 23.228. To this end, until now the I-CSCF unit has generated a Cx query message or Cx select pull message and transmitted said message to the main mobile switching centre (“Home Subscriber Server”). The said messages contain at least the identifier of the S-CSCF unit, provided this is already known to the main switching centre. Further data, for example relating to the resources (“capabilities”) of the S-CSCF unit can also be transmitted in addition to the identifier of the S-CSCF unit, if this is necessary for setting up the desired communication service. The I-CSCF unit carries out a new registration based on the data transmitted. If the identifier of the S-CSCF unit is missing, then the I-CSCF unit selects an S-CSCF unit just on the basis of the resources transmitted.